The present invention relates generally to aerodynamic control systems. More particularly it relates to an aerodynamic lift-generating assembly adaptable to unguided projectile to incorporate a capability for flat trajectory flight.
Requirements currently exist for an unguided projectile having a flat trajectory flight capability. For example, in military applications an anti-tank weapon system is a lightweight, hand-held weapon capable of launching an explosive-laden projectile directly at a vehicle target located at a moderate distance. Preferably, the projectile should possess good armor penetration capability and fly a flat trajectory. Existing weapon systems of this type employ low projectile launch velocities which result in lofted trajectories. This in turn requires the user to make time-consuming range estimates and subsequent launcher adjustments. Unfortunately, these launcher adjustments are inaccurate since it is very difficult to estimate range to better than 30 percent. Thus, this substantially reduces the first round hit probability of the system.
Flattening of the trajectory of a projectile fired from a hand-held launcher can be accomplished by increasing the launch velocity of the projectile and/or by employing projectile lift. There is, however, a limit on the permissible velocity increase because of the recoil factor. For example, if the desired vertical drop of a five-pound projectile at a target range of 250 meters is one foot, then the launch velocity would have to be increased to approximately 3300 ft./sec. This is far in excess of the capability of any existing or planned hand-held launching system. Therefore, until a far superior recoil and blast abatement system is conceived and built, the direct means of achieving a flat trajectory, namely, greatly increased initial velocity, will have to be held in abeyance.
Alternative methods of achieving a flat trajectory include providing the projectile with lift and lift combined with increased initial launch velocity, the latter permitting design tradeoffs between launch velocity and the size of the lifting surfaces required. These alternative methods, however, present their own set of problems. In the design of a lifting projectile, the lift force should be equal to the weight of the projectile, and means must be provided for orienting and maintaining the lift force in the vertical direction.
Lift can be generated easily by using wings, fins, body asymetries, jets, etc. Orientation of the lift vector, however, is a much more difficult problem. Because of manufacturing asymmetries of the stabilizer and of lifting surfaces, the projectiles will develop significant roll displacement in flight. The resulting roll rates can be large since the axial moment of inertia of these projectiles is extremely low, varying in the range of 0.0012 to 0.0025 slug-feet-squared. Controlling the roll of the lift vector, then, is a major problem in the design of lifting projectiles for flat-trajectory flight. In the interest of providing a lifting projectile that is simple and inexpensive, the more costly active roll control systems are not attractive in comparison to the simple, passive and semiactive types of roll control systems. Active control systems employ sensors to produce signals which regulate the control surfaces in response to motion outside of a predetermined, acceptable range. Passive control systems employ fixed-orientation control surfaces which are not self-correcting in response to a motion deviation, and semiactive systems incorporate into the control surfaces a predetermined amount of corrective control, usually via mechanical means.
Methods of controlling or minimizing the roll displacement of the projectile can be categorized as follows: (a) roll minimization, (b) roll resistance, (c) aerodynamic roll stabilization techniques, (d) attitude stabilization, (e) gravity orientation, (f) gyro-orientation, and (g) ballutes and streamers. Each of these methods possesses limitations when applied to roll control of a lifting projectile launched from a hand-held weapon system, such as inadequate or improper roll control, complexity, too expensive, or introduction of other aerodynamic control problems. The prior art is replete with projectiles which apparently possess flat-trajectory capabilities and attempt to obviate one or more of the foregoing problems. For example Detalle, in U.S. Pat. No. 3,752,425, describes a missile having a pair of extended lifting airfoils with a gyroscope acting as a banking stabilizer to maintain a substantially constant attitude for the airfoils. One problems with Detalle's missile is the roll instability introduced by the rigidly-fixed lifting airfoils. In a copending application by C. Kalivretenos and M. Brown entitled "Flat Trajectory Projectile," the foregoing problems are solved in an unguided projectile having a roll-stabilized lifting device which is rotatably supported on the projectile.
There exists in the military inventory a large quantity of unguided projectiles for use with hand-held, tube launcher weapon systems. Since the shelf life of the projectiles is approximately ten years, they are not likely to be discarded soon, and the incorporation of a flat-trajectory capability into these projectiles will greatly enhance their performance and effectiveness. The existing means of achieving a flat-trajectory capability cannot be employed since they are designed to be incorporated into newly-fabricated missiles and projectiles, not to existing, already-fabricated ones.
The invention described herein provides an efficient, effective, economical and simple means of adapting an aerodynamic lift-generating device to existing unguided projectiles to provide them with the capability for substantially flat-trajectory flight. Adaption of the lifting devices require no modifications to the projectiles, and impart no deleterious effects thereto.